1. Field of Invention
The invention relates to a process and a device for detecting electrically conductive particles in a liquid flowing through a pipe by means of eddy currents.
2. Description of Related Art
German Patent Application DE 2 108 717 A1 describes a process and device of the type to which the present invention is directed, two induction coils being located in two branches of an alternating current bridge circuit, the other two branches of which are formed by the halves of another coil. Liquid flows through the coils in the axial direction and the coils can be arranged in succession in the flow direction, the impedance changes which are caused by the passage of the particles and the difference of the impedance change in both coils being evaluated. An arrangement is shown in which the liquid flow is divided into two parallel component sections which each flow through one of the two coils, then an axial offset of the coils is not necessary.
A similar device is described in German Patent Application DE 28 40 358 A1.
The company momac GmbH & Co. KG, 47408 Moers, Germany sells a device under the name “metalscan” in which three coils are arranged in succession in the flow direction, the first and the last coil acting as the transmitter coils and the middle coil acting as the receiver coil to detect passage of electrically conductive particles from a lubricant circuit. The first and the last coil are polarized in reverse.
Other devices in which the signal from the induction coils through which a liquid has flowed is used for particle detection are described for example, in International Patent Application Publications WO 2004/081608 and WO 2004/104561, and European Patent Applications EP 0 778 937 A2 and EP 0 451 209 B1.
German Patent Application DE 39 31 497 A1 discloses a process for inductive detection of particles in lubricants, a coupling coil embedded in a coil through which flow takes place axially being resonantly excited and the passage of particles being detected using the energy removed from the coil system by the eddy currents. In this connection the particle size is determined from the signal amplitude. In order to prevent adulteration of the measurement by the coil sensitivity which decreases in particle passage in the coil middle compared to the coil edge, a swirl generator provides for the particles passing the coil always near the coil wall.
German Patent Application DE 31 17 319 A1 describes detection of the flow velocity of a liquid metal by means of eddy current measurement using a cross correlation function.
German Patent Application DE 40 14 756 A1 describes determination of the velocity of a body or material by means of eddy current measurement, a correlation function being performed.
U.S. Pat. No. 3,575,050 and German Patent Application DE 28 50 246 A1 mention that there are flowmeters based on eddy currents.
A conventional measurement process for nondestructive and noncontact detection of faults in a test specimen, especially a semi-finished metallic article, is induction and measurement of eddy currents in the test specimen. In this connection, the specimen is exposed to periodic alternating electromagnetic fields by means of a sinusoidally energized transmitter coil. The eddy currents induced thereby in the specimen in turn in the coil arrangement which is used as a probe and which can have a single coil (“absolute coil”) or two subtractively connected coils (“difference coil”) induce a periodic electrical signal which has a carrier oscillation according to the transmitter carrier frequency with an amplitude and/or phase which is modulated by the fault in the specimen in a characteristic manner when the fault travels into the sensitive region of the probe, i.e., into the effective width of the probe. The transmitter coil forms the primary side and the receiver coil(s) form the secondary side of a transformer arrangement. One example of this arrangement can be found in European Patent Application EP 1 189 058 A2. If more than two receiver coils are used, this arrangement is also called a “multi-difference coil”.
Conventionally, to scan the specimen the latter is moved linearly with respect to the probe, but also arrangements with a rotating probe being known. The signal detected by the probe is conventionally demodulated in analog fashion, for example, by means of synchronous demodulation, and then, is subjected to evaluation in order to detect faults in the specimen. The signal is digitized conventionally first for evaluation and display of the fault signal, i.e., after demodulation of the coil signal. One example of this process can be found in German Patent Application DE 40 03 330 A1.
Similarly, electrically conductive particles in a liquid which is flowing through the coils cause eddy current losses which in turn are reflected in a measurable impedance change of the coils. In this way, by means of an inductive coil arrangement, electrically conductive particles in a liquid flowing in a pipe can be detected. This is especially advantageous when the concentration of metallic particles in the lubricant circuit of a machine is to be detected in order to draw conclusions about the machine state (the concentration of metallic particles is generally a measure of machine wear).
These eddy current measurement processes are relatively complex and expensive due to the effort required for analog demodulation. Furthermore, it must be considered that ordinarily different filter sets for the demodulated signal are necessary for different relative velocities of the specimen to the probe, i.e., for different ejection or test speeds; this entails additional cost when the specimen velocity is variable.
U.S. Pat. No. 5,175,498 describes an eddy current measurement process in which the measurement signal received by the coil probe is digitized by means of a triggerable A/D converter, and then, in digital form, is demodulated by means of a Fourier transform. Triggering of the A/D converter, i.e., the sampling rate, is controlled depending on the forward velocity of the specimen which has been detected by means of an encoder, in order to avoid errors resulting from backward motion of the specimen in the signal evaluation.
U.S. Pat. No. 4,445,088 describes a magnetic stray flux measurement process in which a metallic specimen is moved relative to a probe, the measurement signal which has been detected by the probe being digitized after passing through a bandpass filter by means of a triggerable A/D converter, the triggering of the converter, i.e., the sampling rate, being controlled by the advance rate of the specimen which is detected by means of a velocity sensor. For fault detection, the amplitude of the digitized signal is evaluated with respect to a threshold value being exceeded, the choice of the sampling rate depending on the test speed being used to achieve a given measurement accuracy which is independent of the specimen velocity.
International Patent Application Publication WO 2006/007826 A1 describes an eddy current measurement process for detecting faults in a specimen moved relative to a probe which is characterized by an effective width with a velocity, by means of the probe a periodic electrical signal being detected which has a carrier oscillation with an amplitude and/or phase which is modulated by a fault in the specimen, the probe signal being filtered and being sampled by means of a triggerable A/D converter stage in order to obtain a demodulated digital measurement signal which is filtered by means of a digital, frequency-selective, adjustable second filter unit in order to obtain a useful signal which is evaluated to detect a fault in the specimen, the A/D converter stage being triggered with a n-th, integral fraction of the frequency of the carrier oscillation, n being chosen as a function of the fault frequency which arises as the quotient of the relative velocity between the specimen and probe and the effective width of the probe, and the frequency-selective second filter unit being set as a function of the fault frequency.